72 VETERINARY PHYSIOLOGY 



From this influence of the electric current upon (a) excit- 

 ability and (6) conductivity certain differences are to be 

 observed in the effects of stimulating an exposed nerve with 

 currents of different strengths and of different direction — 

 downwards, to, the muscle or upwards, from, the muscle. 

 These have been formulated as Pfliiger's Law. But 

 since they have no bearing upon the stimulation of 

 unexposed nerves in animals they need not be considered 

 here. 



(c) When a neuron is stimulated in the middle, the change 

 travels in both directions, although its result is only made 

 manifest by the action of the structure at the end on which 

 it normally acts. (i.) This two-way conduction may be 



Fig. 32. — To show the effect of a gah'anic current upon the excitability 

 (continuous line) and upon the conductivity (broken line) of a nerve. 



demonstrated by using the electric changes as an index of 

 nerve action connecting a galvanometer with the nerve on 

 each side of the point stimulated instead of with only one 

 side, as in fig. 31. (ii.) It is also demonstrated by the 

 experiment of "paradoxical contraction," in which the 

 electric variation, set up by stimulating the branch of the 

 sciatic nerve of the frog going to the muscles of the thigh, 

 stimulates the nerve fibres to the gastrocnemius lying along- 

 side of it, and causes that muscle to contract. That this 

 does not occur when impulses from the central nervous 

 system pass along the nerve is because the strength of these 

 impulses, i.e. the number of fibres implicated, as indicated 

 by the electrical change, is very much weaker than that 

 of impulses caused by direct stimulation {Practical 

 Physiology). 



